Furthermore the invention is directed to a method for operating such a heatable roll.
Heatable rolls are known in the form of rolls with displacement bodies, for example from DE-PS 28 14 244, as rolls with peripheral bores in the roll jacket for the throughflow of a heat carrier from DE-OS 38 38 726, or as rolls with deflection compensation via hydrostatic support elements from DE-PS 35 16 535.
It is known from DE-OS 41 30 672 to cool such heated rolls over their entire length from the outside when the heat generated in the roll interior is no longer led away via the web of material such as is, for example, the case when a tear arises in the web of material. Through this cooling it is intended to ensure that the roll does not heat up too much, which would necessarily have the consequence of a change of the diameter profile.
In the event of web tearing, it is not possible to achieve a situation by interrupting the supply of the heat carrier medium to the heated roll such that a disturbing increase of the surface temperature of the roll does not occur. The reason for this is that the high temperature gradient which exists from the inner side of the roll jacket to its surface means that the roll can only react sluggishly with respect to temperature changes.
It is also already known to encapsulate rapidly running heated rolls in thermal insulation and thus to avoid disturbing heat losses to the environment by convection and radiation during the normal operation of the roll. The presence of such thermal insulation leads, with tearing of the web of material, to an increase in the problem with respect to the temperature increases which occur.